1. Field of the Invention
The present invention relates to turbofan engines and, more specifically, to a high bypass ratio turbofan engine having a low pressure, partially geared counterrotating turbine for driving a booster compressor and a fan section.
2. Description of the Related Art
High bypass ratio turbofan engines, that is, turbofan engines having a bypass ratio of greater than about 8:1, tend to be heavy and expensive because the low speed, low pressure fan drive turbine, also referred to as the power turbine, and the low speed booster compressor each require numerous stages to accomplish the required amount of aerodynamic work as dictated by the thermodynamic working cycle. Also, the low speed booster compressor may be subject to serious icing problems where the speed of rotation is relatively low.
With reference to FIG. 1, prior art high bypass ratio turbofan engine 100 includes a low pressure or power turbine 102 which drives a booster compressor 104 disposed forward of a core section 106, and a fan section 108 disposed forward of booster compressor 104, via a drive shaft 110. Conventionally, in order to reduce the rotational speed of the power from the turbine section 102 to the required rotational speed of fan section 108, a reduction gear 112 is disposed in shaft 110 between booster compressor 104 and fan section 108. In this manner, booster compressor 104 is rotated at the same high rotational speed as turbine 102, and fan section 108 is rotated at a lower shaft speed via reduction gear 112 as required for good fan efficiency.
However, the engine arrangement as illustrated in FIG. 1 suffers from two significant drawbacks. First, reduction gear 112 must transmit the total fan drive horsepower from shaft 110 to fan section 108. Hence, reduction gear 112 is of necessity very heavy and, due to the inefficiency and heat loss inherent in reduction gears, requires a bulky, heavy and inefficient oil cooler. Secondly, because the maximum rotational speed available is limited by turbine blade stress criteria, the number of LP turbine stages must be relatively high in order to maintain efficient stage loading. A large number of LP turbine stages increases weight and rotor dynamics related design problems.